1. Field of the Invention
The present invention relates to a transmitting power control circuit for use in a mobile radio equipment or the like for transmitting power at a predetermined value.
2. Prior Art
FIG. 6 is a block diagram indicating a part of transmitting side of a conventional mobile radio equipment. In this figure, the reference numeral 1 denotes a main apparatus of radio equipment; 2, a transmitting output unit for outputting a transmitting signal of the predetermied level; 41, a control unit which controls starting of transmitting output unit 2; 5, an antenna for sending a transmitting signal; 8, a reference signal generator for generating an output reference value. In addition, 21, is a high frequency signal amplifier (hereinafter referred to as amplifier) for amplifying the transmitting signal to the predetermined level in accordance with an instruction signal; 22, is an output level detector for allowing the transmitting signal of the predetermined level to pass and outputting a level signal in accordance with such predetermined level; 23, is a transmitting output control unit for comparing the level signal and output reference value and applying a current depending on difference of such signals to the amplifier 21 as an instruction signal.
Next, operations will be explained. The amplifier 21 amplifies a transmitting signal in accordance with the instruction signal and outputs a transmitting signal of the predetermined level. The amplified transmiting signal is transmitted from the antenna 5. The level of amplified transmitting signal is detected by the output level detector 22. The output level detector 22 outputs a level signal in accordance with the detected level. The transmitting output control unit 23 compares the output reference value input from the reference value generator 8 and level signal and outputs a difference current to the amplifier 21 as the instruction signal. Accordingly, the transmitting output control unit 23 controls the amplifier 21 so that the level signal matches with the reference value. The output level of amplifier 21 is thus kept constant. Moreover, since the level of transmitting signal of a mobile radio equipment is determined depending on the system used, the output reference value is set so that the output level becomes equal to a level in the specified range.
FIG. 7 is a circuit diagram indicating an example of the transmitting output control unit 23. As shown in FIG. 7, the level signal is input to an input terminal 27. Meanwhile, the output reference value is input to the input terminal 28. The comparator 24 generates a voltage depending on a voltage difference of a couple of input voltages Moreover, a base current flows into a transistor 25 depending on such voltage and thereby resultant collector current flows. Next, the collector current of transistor 25 becomes a base current of transistor 26. Therefore, the transistor 26 allows flow of the collector current depending on the collector current of transistor 25. This collector current flows into the output terminal 29 as an instruction signal. As explained above, the transmitting output control unit 23 applies an instruction signal to the amplifier 21 depending on the level signal in order to control an output level of the amplifier 21.
FIG. 8 shows a relationship between power level and output level of a mobile station of the Advanced Mobile Phone Service: AMPS in the USA. In this case, the power level of eight (8) steps (six kinds) is defined. Accordingly, a reference value generator which generates six kinds of output reference value (values depending on the output level indicated by black points in FIG. 8) is required in place of the reference value generating unit 8. Moreover, the control unit 41 issues an instruction to the reference value generating unit 8 to output the output reference value depending on the power level instructed from a base station.
In an ordinary mobile radio communication system, change of field depending on the location of a mobile station, called fading, is generated. FIG. 9 shows an example of the fading. This profile is observed in the side of mobile station, but when a mobile station is moving, similar phenomenon can also be observed in the base station. In FIG. 9, it is assumed that the receivable field level in the receiving side is A. In this case, if the field level changes according to a field distribution as shown by B with variations from the predetermined transmitting level in the transmitting side by fading, reception becomes impossible in some cases. Therefore, it is required in the transmitting side that a signal higher level than such transmitting level is transmitted in order to obtain a higher level field distribution, for example, C in the receiving side. Here, the mobile station transmits the data signal to be used for incoming and outgoing calls and the voice signal. The data signal is required to show the distribution C of receiving field in order to eliminate error but the voice signal is not always required to show such distribution. For instance, if the receiving field distribution B is obtained, voice communication is not disturbed, although a momentary noise is generated during communication.
In the case of mobile radio equipment, a large amount of power is consumed in the amplifier 21. FIG. 10 shows relationship between the output power of amplifier 21 and current dissipation. For example, when the amplifier 21 is operated at an output of 30 dBm, current dissipation is 600 mA. When a battery with capacity of 0.6 Ah is used, it can serve for an hour. When the amplifier is operated in output of 29 dBm, current dissipation is 500 mA and the battery can serve for 1.2 hours. As explained above, the output of the amplifier 21 is set as low as possible within the specified range in order to reduce power consumption but this will result in a problem due to lower output of amplifier 21 in view of preventing errors in of data signals as explained above.
The existing transmitting power control circuit is structured as explained above. Therefore, an output level of amplifier 21 is constant and such constant value is set to a rather higher value in the specified range to allow transmision of data signals. Since the high output level required for transmission of data signals is used also for transmission of voice signals, problems arise in that current dissipation increases, a large capacity battery is required for long term communication and the apparatus becomes large in size.